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Cadence: Measuring Capacitance of a Node?

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wylee

Full Member level 1
measuring port capacitance

Let say I have a combinational logic gates which have 1 input node and 1 output node.

In Cadence, what method to use to measure the input capacitance (Cin) and output capacitance (Cout) of these two nodes?

lijianheng

Full Member level 2

Maybe SP analysis. You can have a try.

thcm

Member level 2

keep the bias ,then add a ac source using a very large cap connected to the node of interest.

Grig

Full Member level 4

Hi
Cin=imag(Y(1,1))/2/pi/freq
Cout=imag(Y(2,2))/2/pi/freq

gszczesz

Full Member level 2

At the input, put a psin port in series with a VDC.
In the psin properties, set to to be port 1. Optinally set the type from sine to dc and the impedance to be low (i.e. 50Ohms, or 1 Ohm).
Set the VDC value to be a variable, such as "vin".

On the output side, put a psin port to GND and port #2.
Set the psin port impedance to be very high (i.e. 1 Meg). This is so the DC current going through it won't load down the logic gate much. Putting a large series cap (DC blocker type) is risky in that the frequency of your sweep may be affected by it.

Now run a SP analyses over the variable vin. Set the frequency to be very low (i.e. 100Hz) so that parasitic capacitance to substrate won't affect your measurement.

To get the capacitance, plot the ZM(1) for input port or ZM(2) for output port. You can do it through the calculator or the main display dialog. Plot just the imaginary component.

The ZM(1) can be plotted as capacitance by the equation: 1/(2*3.1416*100*imag(ZM(1)))
Where 100 is the 100Hz frequency. If you use something else then change that.

Greg

hannover90

hannover90

Points: 2

Hughes

Increase the input voltage linearly, and measure the input current. Then
C = I / (dV/dt)
Code:
Vin in 0 pwl 0 0.0 10n 2.5
.meas tran imax min I(Vin)
.meas tran Cin param = '-imax / (2.5/10n)'
.tran 10p 10n

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